2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 8
Presentation Time: 8:00 AM-4:45 PM

Whole Rock Major Element Analysis in the Field Using Laser-Induced Breakdown Spectroscopy

LIKES, Tristan and MCMILLAN, Nancy J., Geological Sciences, New Mexico State University, Box 30001, MSC 3AB, Las Cruces, NM 88003, tlikes@nmsu.edu

Minimal sample preparation was sufficient to obtain accurate predictions for major element concentrations in 24 lava samples with varied compositions (45 – 80% SiO2) using Laser Induced Breakdown Spectroscopy (LIBS) and multivariate analysis. Sample preparation consisted of cutting the samples in order to produce a flat surface for laser shots and removing unwanted loose particles and fingerprints with water and ethanol. The LIBS data collection process used an Ocean Optics 2500 LIBS system with a Nd-YAG laser (1064 nm wavelength) to ablate a small amount of material from the sample surface, forming a plasma whose emissions were used to generate a spectrum for the sample by diffracting with a series of seven Echelle gratings in Czerny-Turner design. In order to approximate the homogeneity of powders used in XRF analysis, the data set for each sample consists of spectra from 600 random locations on the sample surface. These were then reduced to six averages of 100 shots each to account for the shot to shot variability inherent in the LIBS technique. A Partial Least Squares (PLS) regression of this LIBS data using multivariate analysis and the program Unscrambler predicted whole rock major element concentrations with reasonable accuracy. Predicted concentrations were compared to those obtained using XRF to produce R2 values of 0.89 for SiO2, 0.77 for TiO2, 0.74 for Al2O3, 0.86 for Fe2O3, 0.78 for MnO, 0.74 for MgO, 0.89 for CaO, 0.84 for Na2O, 0.88 for K2O, and 0.71 for P2O5 (all with intercepts near zero). These results indicate that this method of minimal sample preparation, coupled with short data collection and processing times is ideal for the determination of whole rock major element analysis in the field.